This invention relates to the production of semiconductor materials and structures based thereon and, in particular to a device for epitaxial growing of semiconductor periodic structures from a gas phase.
There are already known in the art devices whose principle of operation is based on epitaxial growing of periodic structures from a vapor phase by quick periodic changes of the concentration of the variable component in the gas flow supplied from the sources of substances towards the substrate.
The known device for epitaxial growing of semiconductor periodic structures from a vapor phase comprises: a vertical tubular reaction vessel filled with gas and provided with an inlet and outlet for said gas containing agents for carrying out the chemical reaction, a first disk located in the vessel and provided with an external surface and an operational surface carrying a substrate, which is mechanically connected to an electric motor for rotation of the first disk, the motor being located outside the vessel and ensuring rotation of the first disk about the vertical axis of the vessel, sources of the substances making up the layers of the semiconductor periodic structure being grown, a mechanism ensuring a specified sequence of the substance transfer from the sources to the substrate, and heaters for the sources and for the substrate, which produce the required temperature difference between the latter.
The mechanism ensuring the specified sequence of the transfer of substances from the sources to the substrate is an injection system placed outside the reaction vessel and provided with a three-way solenoid valve switched by means of an electron time relay.
The injection system of the known device is designed for quick feed of the vapor phase components, their ratio being determined by the position of the three-way solenoid valve regulating the supply of the substances from the sources outside the reaction vessel.
The above mentioned electron time relay of the injection system is intended for control of the switching periods of the solenoid valve and the duration of the injection cycle for supplying the vessel with the gas mixture of each composition corresponding to the composition of the alternating layers in the periodic structure being grown.
It is possible, with the help of the electronic control setting the duration of the injection cycle to regulate not only the thickness of the layers in the periodic structure but also the ratio of the concentrations of the variable component in the alternating layers of the periodic structure.
The known device is characterized by a very small volume of the vessel (about 100 cm.sup.3), which is due to the required quick change of the vapor phase, when switching from growing of a layer of one composition to growing of a layer of another composition.
Another characteristic of the known device is the fact that a part of the sources (those producing highly volatile components of the periodic structure) is located outside the vessel.
The source producing a less volatile component is located inside the vessel and its temperature is maintained by a heater placed outside the vessel. The substrate heater is also placed outside the vessel.
The known device is employed for epitaxial growing of a "superlattice" periodic structure based upon GaAs.sub.1-x P.sub.x solid solutions with variable x values (where x is the mole fraction of gallium phosphide in the solid solution).
The sources of arsenic and phosphorus in the known device are bottles containing PH.sub.3 phosphine and AsH.sub.3 arsine mixed with hydrogen. The arsine flow is constant, whereas the phosphine flow is periodically cut off by means of the three-way solenoid valve.
The source of gallium is metallic gallium placed in a container located in the upper part of the vessel.
In order to effect the chemical reaction of transferring gallium from the source to the substrate, the gas mixture (H.sub.2 + HCl) is fed into the vessel, hydrogen chloride HCl being the chemical agent for the chemical reaction.
The temperature of the gallium source is 900.degree. C and the temperature of the substrate is 760.degree. C.
If the volume of the injection system, supply pipes and the tubular vessel itself is sufficiently small (about 100 cm.sup.3) and the flow rate of the gas mixture through the vessel is sufficiently high (about 1,000 cm.sup.3 /min), the known device permits quick periodic change of the vapor composition in the vessel space and respective alternation of X values from one layer to another during the epitaxial growing of the GaAS.sub.1-X P.sub.X solid solution.
The above described known device has been used in order to grow periodic semiconductor structures with a period within the range of from 225 A to 1,000 A and the amplitude of the periodic X variation (gallium phosphide content) from 0.1 to 0.4 mole fractions.
Layers 110 A thick were grown during 2 second long injection cycles at a velocity of the epitaxial growth of 40 microns/hour, the thickness deviations from its mean value throughout the deposited periodic structure being less than 1%.
The same known device was used to produce a structure based on Cd.sub.x Hg.sub.1-x Te.
Thus, the above described known device can be employed for producing semiconductor periodic structures based on solid solutions of a different nature, though in any case the use of gaseous substances as sources of semiconductor components is compulsory.
However, despite the fact that the known device can be used to obtain a structure possessing required properties and advantages, this technical design known in the art is not devoid of some disadvantages inherent in its constructional peculiarities. The main disadvantage of the known device is the fact that the solenoid valve cannot measure and feed gaseous components in batches less than the amount of the component in the 110 A thick epitaxial layer. The process of batching becomes impossible, when the volume of the gas passing during one cycle becomes comparable to the effective internal volume of the valve itself, which happens when the injection cycle becomes shorter than 2 seconds. In this case the amplitude of the phosphorus periodic variations within the vessel space and in the growing periodic structure becomes uncontrollable and this seriously complicates growing of periodic structures with a period less than 200 A and less than 100 A thick layers, that is qualitatively new "superlattice" periodic structures, where new quantum dimensional effects can appear.
The next serious disadvantage is the low efficiency of the known device owing to the reduction of the vessel volume in order to facilitate periodic changes of the gas phase in the vessel in the shortest time possible during the growth of "superlattice" periodic structures with a period of 200 A and less. That is why only one normal substrate can be loaded into the vessel of the known device with a diameter of 15 mm and, respectively, the process of epitaxial growing of only one periodic structure can be carried out.
Owing to its low output capacity, the known device is unsuitable for industrial production of semiconductor periodic structures on a large scale.
Another important disadvantage of the known device is the restriction of the range of substances producing the components for the periodic structures being grown by the fact that only highly volatile substances and compounds can be used for batching.
These substances and compounds should be synthesized in advance, be pure enough for "the use in electronics" and should not interact with both the material of the solenoid valve and the material of the pipes connecting the sources of highly volatile substances to the solenoid valve and the vessel. This disadvantage restricts appreciably the scope of semiconductor structures which can be used as the base for production of periodic structures.
One more disadvantage of the known device is the fact that it is impossible to control separately the content of the basic components constituting the semiconductor solid solution in the gas phase and the content of doping impurities. This disadvantage does not permit control of the distribution of the doping impurities in the semiconductor structure being grown irrespective of the distribution of the basic components of the solid solution. As a result, the known device permits production of only the most simple periodic structures and can not be used to produce complex structures in which, for example, heterojunctions are combined with p-n junctions or n-n.sup.+ junctions.
It is also a disadvantage of the known device that it permits changing of the gas phase composition only over the entire surface of the substrate and cannot provide control of the main parameters of the periodic structure (amplitude and period of composition change) at definite separate parts of the substrate. The known device, for example, cannot be used to grow periodic structures, wherein the period is changed according to a certain principle from one edge of the substrate to the other.
This disadvantage is accounted for by the fact that the gas phase is changed in the entire volume of the vessel during the process of growing.
Finally, one more disadvantage of the known device is the fact that it permits no fine adjustments in the composition of the periodic structure while switching from one layer to another layer and, consequently, no concentrated profile of a specified shape can be formed. This disadvantage is due to the fact that the process of formation of the concentrated profile, controlled by the solenoid valve switching is superimposed by processes of mixing of gaseous components in the valve itself, in the connecting pipes and in the vessel space. These processes are difficult to control.